Mathematics Education, 2016, 11(2), 377-11
Mental Constructions for The
Group Isomorphism Theorem
Arturo Mena-Lorca & Astrid Morales Marcela Parraguez
Pontificia Universidad Católica de Valparaíso, CHILE
Received 11 November 2015 Revised 12 December 2015 Accepted 01 February 2016
The group isomorphism theorem is an important subject in any abstract algebra
undergraduate course; nevertheless, research shows that it is seldom understood by
students. We use APOS theory and propose a genetic decomposition that separates it
into two statements: the first one for sets and the second with added structure. We
administered a questionnaire to students from top Chilean universities and selected
some of these students for interviews to gather information about the viability of our
genetic decomposition. The students interviewed were divided in two groups based on
their familiarity with equivalence relations and partitions. Students who were able to
draw on their intuition of partitions were able to reconstruct the group theorem from
the set theorem, while those who stayed on the purely algebraic side could not. Since
our approach to learning this theorem was successful, it may be worthwhile to gather
data while teaching it the way we propose here in order to check how much the learning
of the group isomorphism theorem is improved. This approach could be expanded to
other group homomorphism theorems provided further analysis is conducted: going
from the general (e.g., sets) to the particular (e.g., groups) might not always the best
strategy, but in some cases we may just be turning to more familiar settings.
Keywords: APOS theory,
isomorphism theorem
equivalence
relation,
genetic
decomposition,
group
INTRODUCTION
Let
be groups, and let
be a group homomorphism. The
(sometimes called "first") group isomorphism theorem (GIT) states that
is isomorphic to the quotient group
of over the kernel
of f. The GIT is the first of a dozen or so homomorphism theorems that are used to
build groups from others already known, to identify groups and determine their
structure, to examine the solution of equations by radicals, and so forth. These
theorems have immediate extensions to other theories whose mathematical objects
have underlying groups—rings, vector spaces, topological groups, and so on. Thus,
this is a very important subject for mathematics majors.
Mathematical requirements for the theorem are many. Also, from a theoretical
perspective that will be made explicit later on, one should add specific information
on the mental constructions related to each one of those requisites. This refines the
initial view on the complexity of the GIT, but also provides some hope about the
attainability of its learning: in fact, some research has shown that it is not always
Correspondence: Arturo Mena Lorca,
Pontificia Universidad Católica de Valparaíso, CHILE
E-mail: [email protected]
doi: 10.12973/iser.2016.2112a
Copyright © 2016 by iSER, International Society of Educational Research
ISSN: 1306-3030
A. Mena-Lorca & A. M. M. Parraguez
imperative to totally apprehend a mathematical concept in order to understand
another that is built on it (Brown, De Vries, Dubinsky, & Thomas, 1997).
LEARNING OF THE GIT AND RELATED SUBJECTS
Literature on teaching and learning of abstract algebra is scarce as compared to
other areas of mathematics teaching and learning (see, for example, Thomas, de
Freitas, Huillet, Ju, Nardi, Rasmussen & Xie, 2015). Only a few studies concern our
subject, and many are the work of RUMEC-related researchers.1 (Weber, 2002;
Iannone & Nardi, 2002; Novotná, Stehlíková & Hoch, 2006; Ioannou & Nardi, 2009;
Weber & Larsen, 2008; and Larsen, 2009, 2013a) dealt with elementary aspects of
group theory, such as binary operation, subgroup, the notions of homomorphism
and isomorphism; (Larsen, 2013b) added normality and quotient groups; (Ioannou,
& Iannone, 2011) included aids to visualization of cosets and of the TIG. The more
recent works address how group theory concepts are developed from students'
informal knowledge, and include also affective issues. Closer to our approach,
(Asiala, Dubinsky, Mathews, Morics, & Oktaç, 1997; and Lajoie, 2001) are the only
works that we know of that have focused on normality, cosets and quotient groups
and given corresponding APOS' genetic decompositions—to be described later on.
There is consensus among researchers in that, in general, the teaching of abstract
algebra cannot be considered a successful endeavor (Leron & Dubinsky, 1995;
Lajoie, 2001; Godfrey & Thomas, 2008; Stadler, 2011); its difficulty seems to come
from the abstract nature of the objects involved (Hazzan & Leron, 1996).
Furthermore, it may drive off from the subject even students who had thus far been
well motivated (Clark et al., 1997). Dubinsky, Dauterman, Leron, and Zazkis (1994)
have shown that the main problem in this area is that while students must work
with abstract concepts and write proofs, they lean heavily on canonical procedures.
This questions the residual knowledge of the teaching of the subject and shows that
students need more help in learning how to build certain concepts and results and
understanding what they have built (Asiala et al., 1996).
The GIT’s importance notwithstanding, it is common lore that it is a “difficult”
subject: many undergraduate students would agree and many instructors complain
about the low level of results from teaching it. That lore is confirmed by research;
for instance, Brown et al. (1997) showed that most students do not grasp what a
quotient
is nor that its structure depends on the normal subgroup in , and
that they are unable to define functions from
. As shown also by Nardi (1996),
the theorem thus remains unreachable to the learner.
The notions of normal subgroup and quotient group still represent a major
problem for a newcomer, traceable to their introduction by Galois, circa 1830
(Galois, 1897). In 1869, Camille Jordan started a commentary on Galois' work that
he later extended in his book (Jordan, 1869, 1872). That year, Sophus Lie met Felix
Klein in Berlin, and they started a joint work that continued in Paris in 1870. They
were deeply impressed by the work of both Galois and Jordan (Bourbaki, 1999); as it
is well known, Klein used it in the Erlangen Programme and Lie in the creation of
structures nowadays called Lie Groups and Lie Algebras. Nevertheless, Bourbaki
(2006) adds: "Truth to tell, Klein and Lie must both have had difficulty in
penetrating this new mathematical universe," and quotes Klein saying that Jordan's
work seemed to him a book "sealed with seven seals" (p. 287).
Galois' change of the approach to solving equations is far-reaching: he did not aim
directly to find the solutions, but looked for the group structure that they form: if
has a normal subgroup , the equation can be solved by means of and a group
that results from a partition of
induced by
(for him, the group of an
auxiliary equation).
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On the group isomorphism theorem
Burnside (1897) then proved the GIT. This theorem may also serve as a sample of
the process started by Galois: the study of equations reached a trans-operational
level in the sense of Piaget and García (1989), which meant that, from the study of
equations, algebra evolved into the study of (algebraic) structures (Mena-Lorca,
2010). It cannot be claimed that students understand this change of perspective.
(See, for example, Novotná, Stehlíková, & Hoch, 2006; Stadler, 2011).
Now, it can be argued that normal subgroups and quotient groups are
epistemological obstacles in the sense of Brousseau (1998); in particular, the
difficulties that students encounter when studying them are intrinsic to those
subject matters (Mena-Lorca, 2010); and, albeit the concepts nowadays used are
more clear and argumentation is somehow simpler, those subjects remain remote to
the students. Of course, addressing this is not just a matter of going back to
equations, since abstract algebra relates also to other subjects – some of which we
have already mentioned – and, more importantly, because this would tend to
increase the difficulty, since it involves that complex transition of the conception of
algebra itself as a discipline
Accordingly, one should think about a way to relate better this mathematics to
the students' intuition.
Now, the very nature of the TIG is twofold: there is a bijection (a set-theory fact),
and this bijection is an isomorphism (an algebraic fact). Similarly, a quotient
is
a set, which, being normal in , inherits a group structure.
Moreover, it turns out that, if one 'forgets' the algebraic structure, the quotientpartition idea is very simple and customary—not only in the realm of
mathematics—and similarly for a set version of the GIT (see Mena-Lorca, 2010).
Also, provided that the students acquaint themselves with these, and since a priori
there is no reason to expect that a quotient is a group, the need for normality in the
group case might be better understood.
That notwithstanding, the group structure of the quotient may remain hard to
get, but, to approach it, the abelian case—the simplest one—is useless; still, as we
will see, the quotient group
is closer to the students' intuition than the
general case. After examining some cognitive aspects that had to be considered—
and that we will describe later on—this is the way we have chosen.
APPROACHING THE GIT
For the sake of precision as well as clarity, from this point on, unless stated
otherwise,
are groups,
is a group
homomorphism
of
kernel
and
image
Remark: All mathematical concepts and results of this section but the set
isomorphism theorem can be found in (Hungerford, 2003).
Our experience shows that many students learning the GIT proceed according to
their understanding of what "algebra" is and do not address certain key facts. For
example, in dealing with
, they undertake the proof of group properties of the
operation
, but most show no concern about the necessity of independence
of the operation from the representatives taken in the cosets.
We analyzed a number of textbooks currently in use that deal with group theory
(Fraleigh, 2003; Herstein, 1999, Hungerford, 2003, Lang, 2005): when treating
quotient groups, each starts from the construction of the cosets
gathers
them together in
, and then proceeds to the operation
. (See, for
example, Fraleigh. 2003, Dummit & Foote, 2011). We also interviewed researchers
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in abstract algebra that lecture on the subject at the top universities in the country
and found that they proceed in basically the same way.2 Thus, the construction of
quotients is treated as a purely algebraic matter.
On the other hand, it is well known that mathematics (and sciences in general)
permanently and variously uses partitions and equivalence relations (ERs). The
same is true for every individual who uses them extensively in every aspect of his
work and daily life: classifying and considering different objects as equivalent occurs
throughout every day (Mena-Lorca, 2010).
Nevertheless, it seems that when learning about the GIT, students work with
quotients for the first time in a formal way: cosets are presented as a purely
algebraic and abstract notion, and the student has to construct them and then
reunite all of them in another purely abstract object—the "quotient group"—all of
them remote from more familiar objects that they could draw on. Moreover (often
even before comprehending the quotient as a set), they must consider an algebraic
structure on it and define a homomorphism from it. Faced with this situation, they
rush to check properties as we described earlier and do not exhibit an
understanding of the nature of the partition they are dealing with (Mena-Lorca,
2010).
Thus, based on this preliminary evidence, from a cognitive perspective we
expected that, when dealing with our subject, it would be more natural to first build
quotients (i.e., partitions) and then examine the possibility of inducing a structure
on
compatible with the operation on .
For such an approach, we need to consider a theorem that we will call ER/P that
states that every ER defined on a set
induces a partition on
and that,
conversely, every partition on defines an ER on . In this regard, it is interesting
to note that students, even those who lack a precise notion of ER and/or of partition,
spontaneously use an informal version of ER/P (as a theorem in action; see
Vergnaud, 1981), operational to the point that often they do not distinguish between
partitions and equivalence relations defined on a certain set (Mena-Lorca, 2010).
In the algebraic setting, any subgroup
of
defines two ERs on
namely
(left) that induces the partition
and
(right), which in turn induces the partition \
If H is normal in , both ERs coincide, and both partitions coincide—aH being
always equal to Ha. Now,
is, in fact, a normal subgroup of ; the
corresponding
ER
is
,
and
we
have
Thus,
is defined by
, and
simply
gathers together in an equivalence class the elements that have the same image
under ; thus,
. Now, let
be any
sets and
be any function. Then f defines an ER on
by
and it is immediate that
defines a
bijection from
. This last statement we call the set isomorphism
3
theorem (SIT). We can use the SIT as a lemma for GIT. Also, a student may realize
that the SIT is abundantly encountered—for instance, prices in a store are not set for
individual objects but for equivalence classes of objects and so forth.
Our working hypothesis is that the learning of the GIT is simplified and achieved if
the set theoretical and the properly algebraic parts are separated. By this we do not
only mean to consider first the partitions
(whose cosets are characterized by
having the same cardinal number) but, as in the statement of the SIT, any partition
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on the set . Thus, students have the opportunity to draw upon their naive, nonformal knowledge of ERs and partitions and only then, in an already more familiar
environment, worry about the correct definition of operations and functions.
Our study, then, apart from verifying if the approach proposed is suitable, should
also regard: if the students freely use ERs, partitions and the theorem ER/P that
relates them (due to preliminary evidence, we are confident about this); whether
their approach to the GIT can draw on that; and additionally, if, in case they
approach the (re)construction of the theorem in purely algebraic terms, whether
they can succeed in it.
THEORETICAL FRAME
We used the APOS theory that was devised by Dubinsky in the 80s (Dubinsky
1986; Arnon et al., 2014). He bases this theory on Piaget’s reflective abstraction to
describe the construction of mental objects, and he distinguishes different types of
it, or mechanisms: internalization, coordination, encapsulation, generalization, and
reversal. These are the origin of different (mental) constructions: Actions, Processes,
Objects, and Schemas—whence the APOS acronym.
Let us consider a fragment F of mathematical knowledge in Dubinsky’s
perspective (as in Arnon et al., 2014): Individuals have an Action conception of F if
the changes that they make on it are done step by step, obeying stimuli that are and
are perceived as external. Individuals interiorize an Action in a Process concept of F
if they can perform an internal operation that does (or that they imagine does)
essentially the same transformations entirely in their mind, not necessarily covering
all the specific steps. They can coordinate two or more Processes, or reverse one to
obtain a new Process. If individuals think about the Process as a whole and build and
perform transformations on the whole Process, they have encapsulated it in an
Object conception of F. If they need to return from the Object conception to the
Process from which it comes from, they do it by de-encapsulating the Object. A
Schema of F is a collection of Actions, Processes, Objects, and other Schemas that are
consciously or unconsciously related in the mind of the individual in a coherent
cognitive structure. Coherence relates to recognizing relationships within the
Schema, recognizing whether the Schema can solve a particular mathematical
situation, and using it in such a case. In dealing with a mathematical problem, the
individual recalls a Schema and unfolds it to gain access to its constituents, uses
relationships between them, and works with the whole. A Schema is ever evolving
and can be considered as a new Object to which an individual can apply Actions and
Processes; in such a case it is said that the Schema has been thematized.
APOS methodology has been consolidated since the 80s and it is considered a
successful method of research in the area of abstract algebra learning today
(Dubinsky & Lewin, 1986; Dubinsky, 1991; Asiala et al., 1996; Brown et al., 1997;
Trigueros & Oktaç, 2005, Arnon et al, 2014). It uses a Research cycle that aims to
allow for empirical evidence of the mental mechanisms and constructions to be put
into play in the construction of a fragment of mathematics: theoretical analysis,
design, and implementation of instruments and data analysis and verification (Asiala
et al., 1996, Arnon et al. 2014).
A genetic decomposition is a hypothetical model whose aim is to describe both the
mental structures and mechanisms that a student might need—a genetic
decomposition is not necessarily unique—in order to learn a specific fragment of
mathematics (Arnon et al. 2014), and how those mechanisms and structures are
organized in a Schema. It includes prerequisite structures that the students need to
have constructed previously—that is, not just a list of concepts and facts that should
be 'known' by them, but whether they are required as Actions, Processes, Objects or
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Schemas. Specific aspects, such as coordination of Processes, shed light on the
constructions made by the students. Thus, a genetic decomposition not only deals
with the cognitive aspects of the apprehension of knowledge by the students, but
also clarifies the role of its prerequisites—in fact, it is common knowledge that,
when students approach specific subjects, their mathematical prerequisites are not
necessarily well known by them. (Arnon et al. 2014). It typically starts as a
preliminary genetic decomposition, which has to be validated, often after
adjustment based on empirical data (Asiala et al., 1996). A preliminary genetic
decomposition is based on "the researchers’ experiences in the learning and
teaching of the concept, their knowledge of APOS theory, their mathematical
knowledge, previously published research on the concept, and the historical
development of the concept" (Arnon et al., 2014, p. 28).
In what follows, we report some relevant findings on mental constructions
related to groups, normal subgroups, and quotient subgroups obtained by using
APOS theory.
We assume that the student has encapsulated the general concepts of group
(Brown et al., 1997) and of function, epijectivity, and injectivity (Dubinsky, 1991;
Baker, Trigueros, & Hemenway, 2001). Hamdan (2006) presents an initial genetic
decomposition for ERs (and functions). An explicit genetic decomposition for (an
additive group) homomorphism can be found in Roa-Fuentes and Oktaç (2010).
Isomorphism is studied in Leron, Hazzan, and Zazkis (1994, 1995); Leron and
Dubinsky (1995); Dubinsky and Zazkis (1996); and Nardi, (1996, 2000).
Construction and multiplication of cosets is seen in Dubinsky et al. (1994). Asiala et
al. (1997) report at length about the mental constructions on cosets and normality:
roughly two thirds of the students considered were successful in constructing cosets
and normality, while for quotient groups the rate dropped to around one third and,
generally speaking, a high percentage of the students seems lost and can end up
disconnecting from the subject (see also Ioannou & Nardi 2009). In fact, some
researchers doubt the permanence in time of the quotient concept (Dubinsky et al.,
1994). Note that if the encapsulation of the quotient-partition is not achieved it is
certainly doubtful that the concept has some value for the student. Dubinsky (1986)
showed that students’ difficulties with mathematical symbolism (e.g., multiplying
cosets
) come from trying to apply labels before achieving encapsulation.
We found no genetic decomposition for the GIT.
CONSTRUCTING THE GIT
We turn now to requirements for achieving an Object conception of the GIT.
According to the research cited above, one needs Object conceptions of set (for
group), multiple quantification (for isomorphism), function (for homomorphism),
group (as a homomorphism is an Action on a group), subgroup (examining
normality requires its encapsulation), and homomorphism (examining bijectivity
requires its encapsulation). We also need Object conceptions of
(to get
) and
(to compare it with
). A Schema conception of
group consisting of Object conceptions of set, group, subgroup and function as well
as a Process conception of binary operation (the coordination of set and binary
operation plays a fundamental role in the Schema) are also necessary. A genetic
decomposition for the GIT does not need the general concept of quotient group, but
of
, and working with this particular case might help the construction of
the general case. Also, the encapsulation of cosets just as sets does not need to be
carried out by an “algebraic” process: the encapsulation of a quotient set is first
achieved as the partition
defined by
and then
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On the group isomorphism theorem
defined as an operation on
As shown in the research cited above, this is
not an easy task, but in this approach students are able keep in mind that they are
dealing with (sub)sets of elements to start with. Moreover, we did not require an
Object conception of
but a Schema composed of Object conceptions of
group, subgroup (and set, function, and binary operation), and (the naive version of)
ER/P.
Over those constructions, the student is supposed to make a number of
coordinations—several of them already established in the literature reviewed—
such as (in this case,
): subgroup with cosets (as
classes in a partition) through the normality property in order to obtain a Process
conception of a normal subgroup; group with E/R, and subgroup and quotient group
in order to obtain a Process conception of
; function and binary operation, so as
to construct the homomorphism as a Process which encapsulates by means of the
identification of the quotient group and its image; function with ER/P in order to
define a function from the quotient, then a bijection and obtaining the SIT; etc.
METHOD
We designed and implemented some instruments to explicitly reflect the
constructions through which students can grasp the concepts that are needed. For
this, we framed our preliminary genetic decomposition using a case study
methodology (Goetz & Le Compte, 1988; see also Arnon et al., 2014) so as to
provide, through a purposeful selection of participants, knowledge about the
learning of the GIT through the study of mental mechanisms and constructions
shown by comparatively advantaged individuals in dealing with our subjects.
We started with four groups of students, 17 in total, each group from a different
university in Chile. The universities were chosen so as to ensure: equivalence in
admissions processes in terms of basic skills for addressing the mathematical field,
existence of both an undergraduate program in mathematics teaching that included
group theory, existence of a graduate program in mathematics, and affordability for
the study. The students were required to have approved a course in abstract
algebra.
We designed a questionnaire (see Appendix) to be administered to the
aforementioned students. The answers to the questionnaire were not aimed at
determining which students might proceed with the construction of the GIT, but
rather which might not. It was applied in a classroom at the location where the
students' programs are offered and lasted about one hour. At least one of the
researchers was in the room, and provided explanations about symbols or related
questions (e.g., the meaning of “inside” and “outside” mathematics in Question 4 of
the questionnaire) but not prompting.
Then we applied two semi-structured interviews to students chosen based on
their answers to the questionnaire. This type of interview is customarily used in
order to be able to interact with the interviewee and to be able to clarify answers if
needed (Dubinsky et al., 1994). For the seven students selected in Group 1 (Juan,
José, Pedro, Luis, Jorge, Ana, and Manuel), who we could not assume had paid
attention to equivalence relations and partitions, we used Script 1 (see Appendix).
For the three students in Group 2 (Carlos, Marta, and Leslie) we used Script 2 (see
Appendix), since we knew that they had treated equivalence relations and partitions
in their curricula. The aims of these interviews were to gather information about the
viability of our preliminary genetic decomposition and to refine it if necessary, as
well as determining the difficulties encountered by students in dealing with our
theorem.
The interviews were individual, were held in isolated rooms at the location of the
academic units to which the students belonged, and were videotaped. Each
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interview lasted an hour and a half and in all but one of them there were at least two
researchers present. The whole team reviewed the videotapes.
Questionnaire
Given the limited aim of the questionnaire, it was not tested beforehand:
students' inability to proceed with the construction of the GIT would be shown in
the corresponding interviews. Thus, we do not claim that students who did not do
well in the questionnaire would not be able to end up constructing the GIT, but that
we doubt it, and that it was preferable to interview others.
There were two kinds of questions (enumeration refers to the questionnaire):
A. The students should:
 distinguish between elements of a group and sets of them (1a, b, c): if they do
not, they would not be able to consider cosets as elements not of but of a
quotient of which is crucial for defining the isomorphism of the GIT;
 realize that the operation of a subgroup is the restriction of the operation on
(1d): otherwise, since it is needed that the operation on cosets of has to be
induced by the operation on , the students would lose track of the structure
they have to deal with;
 have an idea, though vague, of normality (2a): if not, it is unlikely that they
would be able to approach the need for the operation in the classes to be well
defined;
 be acquainted with the importance of ERs and partitions inside and outside
mathematics, or at least be able to come up with some examples (4): this is
needed to take advantage of the set isomorphism theorem for the construction
of the GIT;
 be able to at least generally express the notions of ER and partition or produce
an approach to them, even in an informal way (5a, b): also to make use of the
set isomorphism theorem;
 realize or consider that there is a connection between ERs and partitions
defined over the same set (5c): the GIT needs for a translation of the ER
defined by
on into cosets defining
;
 be able to define homomorphisms in “simple” cases (6): even though this does
not guarantee to be able to define an isomorphism starting from a quotient, its
absence suggests that the students will not succeed in this last one;
 realize that the commutative property is “preserved” by isomorphism (7):
thinking that a non commutative group may be isomorphic to a commutative
one suggests that the acting idea of isomorphism is only related to bijection
and not to the group structure.
B. We would welcome (but not require) that the student:
 have an idea about why operations on cosets work (2b);
 have a possibility of analyzing normality for subgroups of a small group and
(even) of the construction of a quotient group (2b);
 show an idea about how to identify isomorphic groups that are not stated in
the same way (8).
At this stage, we valued any arguments, even if not formally stated. Based on
experience and research, our expectations for the answers were rather low, and we
did not ask for the GIT.
We collected the questionnaires and tabulated them in a colored matrix in order
to be able to easily glance at the correctness of the answers of each student to each
question. We then decided which individuals to interview by means of identifying
correct answers in most questions other than 2b, 3, and 8; “good” answers on these
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three questions would prevail over some wrong answers in the others. We did not
proceed using a checklist, nor did we hypothesize that the selected students had
clarity on the requisites stated above as that would be determined in the interviews.
Interviews
We did not wait for interviewees to fluidly answer our questions, which would
have been contrary to the existing research as well as our own experience. As stated
earlier, the scripts focused on examining the eventual presence of mental
constructions and mechanisms that would come into play in connection with the GIT
as we propose it: on the one hand, some aspects of group theory, which we would
require even in a rather vague form, and, on the other, questions related to ER/P.
The scripts were used as a broad guide for the researchers' APOS semi-structured
interviews. Notations were explained as needed. Interviewees were asked to explain
their thoughts, to draw, and to give examples. The tone was conversational.
Script 1 began by asking about the GIT and we did not expect a clear answer on
it—in fact, our experience suggested that most students would have forgotten about
it, a result that was shown in the interviews. We then asked interviewees to
elaborate on ERs and partitions. The key issues in our approach were that we
expected subjects to realize were that, given
a function, defines an ER
and that there is a one-to-one correspondence between the quotient
and
. We asked this, but with low expectations. Thus, if students in Group 1
were not able to do this, we gave them the definition of
in order to see whether
they could elaborate from there, that is, relate it to the quotient defined by
on and thus proceed to the GIT.
Script 2 was prepared for interviews that we expected to proceed in a more
direct course.
ANALYSIS AND DISPLAY OF SOME RESULTS
As expected, every interviewee showed Process conceptions of group, subgroup,
and normal subgroup. Luis, Ana, Manuel, and Marta manifested Object conceptions
of group, subgroup, and normal subgroup and were able to construct a quotient
group. Pedro, Juan, and José could not progress much in their interviews and they
could not elaborate on examples nor on drawings: Juan and José tried instead to
recollect definitions (such as Kernel, image, partition) and theorems (such as
). On the other hand, everyone used ER/P as a theorem in
action: when arguing, they changed from ERs to partitions and vice versa. Group 1
students could not elaborate much on the GIT, as expected; Ana was an exception.
Students in Group 2 tended to be more assertive in stating the GIT and reasoning
about it. We will turn to these later.
Coordination
We were able to verify the coordination that we expected. We give some
examples and brief comments:
Subgroup and quotient group. José drew the usual picture of a set, labeled it
,
and wrote
. On the contrary, Jorge did not: he stated
"A normal subgroup
of a group
is a group with certain properties." As we
expected, Jorge could not elaborate on quotients.
Normal
subgroup
and
quotient
group.
Ana
correctly
wrote
; and added: "They serve to construct the
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quotient groups. It is known that if is an abelian group and is a subgroup of
then is a normal subgroup." (We will turn to her later).
Function and homomorphism. The absence of this coordination seems to impede
the construction of the GIT. For instance, Pedro wrote
. We then had the following exchange:
Pedro: But ... the interesting [point] is that I need the function to be bijective in
order to obtain an isomorphism.
Interviewer: You told me that you cannot prescind from the function concept.
Pedro: Oh, but I can prescind from the group concept.
Interviewer: Why?
Pedro: Why can I prescind from the group concept? Because the isomorphism
concept is more general, uh...
On
the
contrary,
Luis
wrote,
not
quite
correctly:
Nevertheless, his explanation showed just a Process GIT. Thus, the coordinating
function and the homomorphism property would not suffice for an Object GIT—as
one might expect.
Normal subgroup, quotient group, and homomorphism. We think that
coordinating these Processes allows getting close to Object GIT. Manuel correctly
stated the GIT, wrote it, and explained:
even,
odd,
and
then
wrote
. Nevertheless, he could
not elaborate on ERs and partitions and he could state neither the SIT nor the GIT.
Suitability of the Genetic Decomposition
Ana's work. Ana, in Group 1, showed clarity in several aspects of the GIT. She
was interviewed again with Script 2 in order to determine whether she could
complete her approach but she could not. She correctly stated the SIT, but when
asked about an eventual relationship with the GIT, she added, “The guarantee that
one has in sets is that in defining the equivalence relation, partitions can be known
and one can easily see the quotient, while for groups we do not have the equivalence
relation.” Thus, she could not coordinate the SIT with the structure of the group.
Carlos's work. Carlos, when asked for examples of ERs, both in mathematics and
in real life, easily provided several ERs and partitions. Questioned on it, he stated
ER/P and explained how to define an ER from any partition. He was then given a
diagram where two sets,
,
are shown and was asked to draw a
general function on it. He filled in the picture as shown in Figure 1.
Figure 1. Carlos's work ("ssi" stands for "iff").
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© 2016 iSER, Mathematics Education, 11(2), 377-393
On the group isomorphism theorem
Asked whether he saw an ER in the picture, Carlos wrote below it as shown in
Figure 1, which he correctly explained. When we inquired whether he saw another
function in the picture, he wrote
being the partition
induced by . He explained this correctly, although he called
“a partition,” as
often happens. In answering what kind of function is g, he said that it is injective and
that it is epijective over its image.
Turning to groups, we had to remind him what
means and we gave him
the definition of
. He checked the ERs properties. When asked, he wrote the
definition of
. His response to our subsequent inquiry about the
corresponding ER is shown in Figure 2.
Figure 2. Carlos's work. (In the third column, he checked what he claimed in the
second).
We then asked for
and he gave this response, “Two elements are
related when they have the same image, which was what we did at the beginning.”
He explained that the SIT and the GIT are quite similar, the difference being the need
for group homomorphisms.
Thus, although Carlos could not work out the GIT by himself, it is clear that if he
was given some definitions, he had a clear view of what had to be done. Moreover,
he was able to argue using one figure where both the group and the quotient
coincided, which is one of the problems that students have to overcome when
reasoning about the GIT.
Carmen's approach. Carmen completely showed that our preliminary genetic
decomposition works: her coordinations were readily apparent. Asked for an RE in
either mathematics or in common life she immediately said, “What catches my
attention more is the one of the pre-images.” She wrote
, and
stated that an ER is defined by inverse images of elements of . Asked for more
examples, she gave one about buses with a common route and added, “If we think as
a partition it comes out much more easily.” She stated and explained ER/P. We then
gave her a diagram,
,
and asked for a general function, and
her response is as shown in Figure 3.
© 2016 iSER, Mathematics Education, 11(2), 377-393
387
A. Mena-Lorca & A. M. M. Parraguez
Figure 3. Carmen's work. (Patentes: license plates; Recorrido: route)
When asked for another function that she may perceive in that setting, she
explained that, for example, bus fares are not fixed for individuals but for sets: buses
on the same route have the same fare. We then asked her to generalize what is
occurring, that is, a sort of theorem that can be stated. Carmen then correctly stated
our SIT. When we talked about a homomorphism between two groups, she wrote
“
”. She remembered that the definition of
was
, and stated that the equivalent classes in
are
. She also wrote
,
and stated that it is an isomorphism. We then had the following exchange:
Interviewer: This theorem is called the GIT. What would you call to the one that
you made at the beginning?
Figure 3. Carmen's work. (Patentes: license plates; Recorrido: route)
Carmen: Phew!... of the isomorphism because it has to have a form... Here there is
no form since there is no structure... but what could it be; in lieu of isomorphism let’s
talk about the theorem of bijectivity.
We asked her about our approach, the SIT, and then the GIT, she responded, “It
seems to be a good idea to me. This theorem could be better understood if one
looked at it using the example of the buses, surely.” When asked whether something
must be taken care of in the group case, she pondered it, and said, “Ah! That it were
well defined,” and then explained this, both expeditiously and clearly.
DISCUSSION
As stated in our Approaching the GIT, we expected that students would freely use
ERs, partitions, and the ER/P theorem even if they could not formally explain them,
and all of them did.
The students who showed an approach to the GIT purely in terms of abstract
algebra were not able to develop it and could not explain it. Those who at first
388
© 2016 iSER, Mathematics Education, 11(2), 377-393
On the group isomorphism theorem
seemed able to (re)construct the GIT but in the end could not, had trouble when
arguing by means of ERs, partitions, and/or ER/P. Conversely, as shown by Carlos's
and Carmen's cases, those who could elaborate on ER/P were able to construct the
SIT and the GIT and also worked faster, had a clearer view, and seemed more
confident. In addition, they were able to construct the GIT from the SIT as we had
expected.
Carlos's and Carmen's cases also show that for the GIT it is not necessary to
construct the general quotient
for
, but rather to realize that one must
have
in order for
to be a group. When students then have to
identify the cosets
, they already have a glimpse of what they are:
just the elements of whose image by is the same than that of . Thus, we think
that a student might benefit from proceeding from
to the general
quotient group
both normality and the good definition
are
easier to deal with if
, and this case is one step further than the abelian
case.
Having shown that our approach to the GIT is suitable, we feel it is appropriate to
try teaching the (first) group isomorphism theorem the way we proposed here in
order to gather further data. Furthermore, Mena-Lorca (2010) has shown that of the
dozen or so group homomorphism theorems mentioned at the beginning, all but one
have corresponding set theory theorems at their bases, which suggests that a
general (categorical) perspective might be used for treating them. In fact, when
proving homomorphism theorems for rings, for example, it is natural to look to the
corresponding group homomorphism theorem, observe that the group
homomorphism is (well) defined, and then just prove its compatibility with the
product. Mutatis mutandis, the same is true for group and sets, respectively. This
should be further analyzed from a cognitive perspective: going from the general
(e.g., sets) to the particular (e.g., groups) might not always the best strategy. For
instance, the GIT is an immediate corollary of the fundamental homomorphism
theorem for groups (that is, if
then every group homomorphism
such
that
,
factors
uniquely
as
,
where
are defined by
respectively). Nevertheless, in some cases, such as the GIT, we may just be turning to
more familiar settings.
NOTES
1. Research in Undergraduate Mathematics Education Community (1995-2003),
founded by Ed Dubinsky.
2. Although they stressed that their teaching preference was to go from the
particular to the general, none of them would try first the “fundamental
homomorphism
for
groups”:
the
universal
property
for
.
3. We did not know beforehand that Neubrand (1981) offers a mathematically
similar approach, although limited to the GIT.
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APPENDIX
Questionnaire
(" " stands for subgroup, is the equivalences class of z in Z.)
1. Which of the following are true, if any?
a.
; b.
; c.
; d.
2. Let
a. Assume that
show that
.
b. On
define
and explain why it works correctly.
3. Let
,
.
a. Show that it is not true that
; b. Examine whether
; c. Show
that G/K is not a group.
4. Give five examples of ER for each:
a. Inside mathematics; b. Outside mathematics
5. Explain:
a. What is an ER; b. What is a partition;
c. A result that links ERs and partitions defined over the same set.
6. Define, if possible, a homomorphism.
a. From
to ; b. From
to ; c. From
to
7. Determine whether
8. Let
be the group of symmetries of a triangle. Determine whether the
subgroup of rotations is isomorphic to some
.
Script 1
1. Let G be a group,
A. State the GIT.
B. i. Explain what normal subgroups are needed for.
ii. Describe the quotient group
.
2. State a theorem that links REs and partitions on a given set. Explain.
3. Let
a function.
i. Show that defines an ER
ii. State a theorem that relates the quotient-partition
with another set.
4. A. (If i. is achieved)
i. State a relationship between the theorem in 3.ii and the GIT.
ii. Explain what is lacking in 3.ii to be the GIT.
iii. Try to fill in what is lacking.
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On the group isomorphism theorem
B. (If 3.i is not achieved)
i. Let
a. Show that
is an ER.
b. What is
?
5. Assume now that is a group.
A. Explain how does
relates to
B. i. Explain the following:
.
.
ii. What conclusion may you extract from i., related to the GIT?
6. Can you distinguish two parts in the GIT?
Script 2
1. State the GIT. Explain and give an example.
2. What does it mean that a subgroup is normal in a group? What is the purpose of
defining normal subgroups?
3. State a theorem that links ERs and partitions defined on a set.
4. Let
be a function.
a. Show that defines an ER
on
b. State a theorem that relates the partition
with another set.
c. State a relationship between b. and the GIT. Try to fill in what is lacking.
d. Show that
defines an ER
. What is
e. Assume now that
is a group homomorphism. How does
relates to
f. Recall the ER defined on
by
. Explain
.

© 2016 iSER, Mathematics Education, 11(2), 377-393
393